1. Field of the Invention
The present invention relates generally to a system for controlling the temperature of exhaust emissions from an internal combustion engine. More particularly, the present invention is directed to a system that uses a heat exchanger for controlling the temperature of the exhaust emissions from an internal combustion engine delivered to a catalytic converter so as to increase the conversion of NOx pollutants by a catalyst to harmless by-products.
2. Discussion of the Related Art
There is increasing interest in diesel engines and other lean-bum engines for automotive application because of their inherently higher efficiency and lower carbon dioxide (CO.sub.2) emissions relative to a conventional gasoline engine. In general, all internal combustion engines which bum various fuels, such as diesel and gasoline, with air, typically produce undesirable exhaust emissions and particles which may be harmful to the environment. These by-products of the combustion process include unburnt hydrocarbons, carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO.sub.2), soot particles, reactive organic gasses, etc. In view of these environmental concerns, most modem vehicles are equipped with an exhaust system having a catalytic converter which functions to reduce or significantly eliminate such exhaust gas pollutants.
Nitrogen monoxide and nitrogen dioxide, generally referred to in the industry as NOx, are considered to be the most difficult exhaust emissions to reduce or eliminate. This is especially true in lean-bum engines, such as diesel engines, two-stroke engines, lean-bum gasoline engines, etc., because these types of engines use excess air to bum the fuel. It is difficult to eliminate NOx emissions in the presence of air, and in order to reduce the NOx emissions, it is necessary to convert the NOx pollutants to harmless by-products using a lean NOx catalyst positioned in the exhaust path.
The use of a Lean NOx catalyst to convert NOx exhaust emissions from internal combustion engines into harmless by-products is well understood. Typically, the catalyst is provided within a catalytic converter positioned in the exhaust path such that the exhaust gasses are forced through and around the catalyst. Unfortunately, a problem exists with known lean NOx catalysts which reduces their ability to convert the NOx pollutants. Unlike conventional three way catalysts used in conventional gasoline engines, a lean NOx catalyst works most effectively within a specific, and somewhat narrow, operating temperature range. For example, the catalyst platinum-alumina is only effective to convert NOx emissions when its temperature ranges between 180-220.degree. C. Similarly, the catalyst copper zeolite is only effective to convert NOx emissions when its temperature is in the range of 380-420.degree. C. The temperature ranges are not exact because the catalyst does not work the same at each temperature in the range. Particularly, the effectiveness of the catalyst falls towards the ends of the range in a "bell" shaped manner. Other catalysts are also known, but also have limited operating temperature ranges.
In addition to this specific problem of NOx conversion, in any type of engine with a conventional catalytic conversion system, the heated exhaust gasses from the engine are used to heat the catalyst to its operating temperature. The exhaust temperature can vary under different vehicle operating conditions. However, this presents a problem because if the engine is not operating at or near its normal operating temperature, the catalyst will be too hot or too cold, and will not operate effectively. For example, at vehicle start-up, the catalyst is typically at ambient temperature and the exhaust emissions are not yet at the normal operating temperature. In contrast, higher load demands on the engine, such as from pulling a trailer, going up hill, operating in the desert, etc. typically cause the exhaust temperature to increase. Continued overheating of the catalyst can cause the catalyst to break down and reduce its normal life. For this reason, catalytic converters are usually positioned farther from the exhaust manifold than desirable for effective pollution control during vehicle start-up. In this position, the catalyst takes longer to heat up to its operating temperature range at vehicle start-up.
Different techniques can be used to solve the above discussed conversion problem. For example, it has heretofore been known to combine different catalysts having different operating temperature ranges into a single catalytic converter for increasing the operating temperature range of the catalyst. Additionally, research continues to find a catalyst which operates optimally over wider temperature ranges. Thus, it is an object of the present invention to provide a system which controls the temperature of the exhaust emissions from an internal combustion engine to maintain the temperature of the catalyst within its operating temperature range that includes improvements over the prior art.